Fix a deadlock in CmdStream.

As per FIXME, CmdStream could have deadlocked if a command printed
enough on stderr. This commit fixes that, but still keeps all of
the stderr output in memory.
Comment to explain CmdStream
Merge remote-tracking branch 'robryk/cmdstream-deadlock'
Comment CmdStream a little bit to facilitate future fixes and contributions
FIXME: implement a timeout in TestCmdStreamLArgStderr to avoid blocking the whole test suite when it fails